Image sensors find applications in a wide variety of fields, including machine vision, robotics, guidance and navigation, automotive applications, and consumer products. Imaging circuits often include a two-dimensional array of photosensors each of which forms one picture element (pixel) of the image. Light energy emitted or reflected from an object impinges upon the array of photosensors and is converted by the photosensors to electrical signals. The individual photosensors can be scanned to read out and process the electrical signals.
One class of solid-state image sensors includes an array of active pixel sensors (APS). An APS is a light sensing device with sensing circuitry inside each pixel. Each active pixel includes a sensing element formed in a semiconductor substrate and capable of converting optical signals into electronic signals. As photons strike the surface of a photoactive region of the solid-state image sensors, free charge carriers are generated and collected. Once collected, the charge carriers, often referred to as a charge packet, are transferred to output circuitry for processing.
An active pixel also includes one or more active transistors within the pixel itself. The active transistors can amplify and buffer the signals generated by the light sensing element. Thus, in contrast to charge coupled devices (CCDs) and metal oxide semiconductor (MOS) diode arrays, an APS can convert the photocharge to an electronic signal prior to transferring the signal to a common conductor that conducts the signals to an output node.
APS devices can be fabricated in a manner compatible with complementary metal oxide semiconductor (CMOS) processes. Compatibility with CMOS processes allows many signal processing functions and operation controls to be integrated on an APS chip. Use of CMOS circuitry with APS devices also reduces the costs of manufacturing. CMOS circuitry also allows simple power supplies to be used and can result in reduced power consumption. Moreover, the active pixels of APS devices allow non-destructive readout and random access.
In an exemplary CMOS APS, charge carriers are collected in the photosite via a photogate. The charge packet is stored in spatially defined depletion regions of the semiconductor, also known as potential wells, in the semiconductor substrate beneath the photosite. The charge packet then is transferred to an isolated diffusion region via a transfer gate. The diffusion region receives the charge from the photogate well and sends a corresponding electrical signal to the pixel amplifier for further processing.
The near-surface potential within the semiconductor can be controlled by the potential of an electrode near the semiconductor surface. If closely-spaced electrodes are at different voltages, they will form potential wells of different depths. Free positive charges (e.g., holes) move from a region of higher potential to a region of lower potential. Similarly, free negative charges (e.g., electrons) move from the region of lower potential to the region of higher potential.
Typically, a CMOS active pixel array is operated in a rolling shutter mode in which each row of the array is exposed at different instants of time. The non-simultaneous exposure of the pixels can lead to image distortion, for example, when there is relative motion between the imager and the image that is to be captured. Furthermore, although the exposure time generally is defined by the duration for which the photogate is turned on, floating diffusion regions can continue to collect photocharges even after the photogate is turned off. Transfer of such unwanted charges into the sense node can result in image distortion and excess noise. Furthermore, the distortions tend to become more pronounced as the exposure time is reduced.